DROUGHT RESISTANCE OF Sorghum bicolor. 4. HORMAL CHANGES IN RELATION TO DROUGHT STRESS IN FIELD-GROWN PLANTS

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Abstract
Hormonal changes in relation to drought stress were studied in 2 cultivars of S. bicolor L. Moench. The cultivars, M-35 and NK300, were grown in a field plot protected by a rain shelter. Plants in one soil compartment were stressed by withholding water while those in the other (controls) were irrigated frequently. Levels of the plant hormones abscisic acid (ABA), phaseic acid (PA) and IAA measured by high-performance liquid chromatography (HPLC) were determined in the youngest leaves of control and stressed plants at intervals throughout the growth cycle. Plant height, senescence, and leaf water status were also determined. Leaf water potential (.psi.w) and solute potential (.psi.s) were reduced in both cultivars by drought stress; values for cv. M-35 plants were lower than cv. NK300. Leaf senescence was higher in M-35 plants and was promoted by stress in both cultivars. Cv. M-35 behaved as a drought-tolerant plant whereas cv. NK300 behaved more like a drought avoider. ABA levels were higher in cv. M-35 control plants than in corresponding cv. NK300 plants and levels in both cultivars followed seasonal changes in leaf water potential. Under drought stress, ABA levels increased between 1.5 and 2 times in both cultivars with the largest increases occurring during the vegetative stage in cv. M-35 and during the flowering stage for cv. NK300. PA levels in both cultivars were higher in stressed than in control plants. PA levels in cv. M-35 plants were relatively low and constant throughout the life cycle, whereas in cv. NK300, levels were high until shortly before flowering. IAA levels were higher in cv. NK300 than in cv. M-35 plants, particularly during the vegetative stage. Under drought stress, IAA levels were reduced in both cultivars with a more pronounced reduction in cv. NK300. The high level of ABA in the more drought-tolerant cv. M-35 was associated with low leaf .psi.w and .psi.s and high leaf senescence. In the drought avoider, cv. NK300, high levels of IAA and PA were associated with high leaf .psi.w and .psi.s. These cultivars, which differ in their response to drought stress, can be distinguished by their leaf hormone levels.